(1) Field of the Invention
The present disclosure pertains to an image forming device capable of both-side printing, such as a printer or a photocopier, executing a printing process by statically transferring an unfixed image onto a recording sheet and then thermally fixing the unfixed image onto the recording sheet. Particularly, the present disclosure pertains to a technology for controlling transfer voltage applied for the static transfer of the unfixed image during both-side printing.
(2) Description of the Related Art
An image forming device such as a printer, a photocopier, and the like is commonly provided with a both-side printing function of statically transferring an unfixed image to each of a first side (one side, e.g., a front side) of a recording sheet and a second side (the other side, e.g., a back side) of the recording sheet and then thermally fixing the unfixed images onto the recording sheet.
Also, a fixing temperature required for thermally fixing an unfixed image onto a recording sheet varies under different conditions, such as an amount of toner to be fixed onto the recording sheet, a type of image to be formed on the recording sheet, and the like. In order to prevent insufficient fixing under these different conditions, a target fixing temperature at which a surface temperature of a heating rotating body is to be maintained during thermal fixing with respect to a recording sheet page may be set to a temperature sufficient to ensure good fixing under printing conditions requiring the greatest amount of heat to be applied for the thermal fixing.
However, when the target fixing temperature is set as described above, more electricity than necessary is consumed particularly when thermally fixing an unfixed image on a page that does not require the greatest amount of heat, which is not desirable for energy conservation.
Technology for reducing the electricity consumption of thermal fixing has been proposed, such as Patent Literature 1 (Japanese Patent Application No. 2012-118496), which discloses changing the target fixing temperature at which the surface temperature of the heating rotating body is maintained during thermal fixing for each page, in accordance with the image content of the respective page. This enables adjusting the fixing temperature to an optimal temperature that is in accordance with the image content of a page that prevents the fixing temperature applied from becoming excessive or insufficient, which in turn reduces the electricity consumption required for thermal fixing.
As described above, the technology described by Patent Literature 1 varies the target fixing temperature between pages. Due to this, particularly when continuously printing two or more pages, the fixing temperature may change by a great amount while performing thermal fixing with respect to one page. In such circumstances, when the two or more pages are two sides of one recording sheet with respect to which both-side printing is performed, the amount of water contained (water content) in the recording sheet may vary in a sheet passing direction due to the change in fixing temperature occurring while thermal fixing is performed with respect to one side. As a result, electrical resistance in the sheet passing direction changes, which produces transfer unevenness when statically transferring an unfixed image onto the other side. This results in deterioration of image quality of the other side.
In consideration of the above-described problem, the present disclosure aims to provide an image forming device having a both-side printing function enabling, during both-side printing, prevention of transfer unevenness caused by change in water content in a recording sheet.